A conventional optical pickup apparatus usually effectuates the recording, the reproduction, the erasing and the like, of information by a single laser light source. However, the above apparatus presents the disadvantage that when recording or reproducing information, the processing time for writing or reading successively information on a recording medium through the use of a single light spot is generally long.
Some of the recent optical pickup apparatuses, in order to shorten the information processing time, are provided with plural sources of laser lights of different wavelengths of oscillation and plural light receiving elements. In such an apparatus, the recording, the reproduction and the like of signals is effectuated simultaneously at different locations on one and same recording medium. Namely, the laser lights emitted from the different laser light sources are irradiated simultaneously on one and same recording medium at different locations, then the different light receiving elements detect simultaneously the reflected lights of the laser lights on the recording medium.
An example of this type of optical pickup apparatus is illustrated in FIG. 4. In this optical pickup apparatus, the laser light 29a emitted from the laser light source 21a in the center-right part of the figure, is transformed into a parallel pencil of rays by a collimating lens 22a, transmitted through a polarized beam splitter 23a, converged by an objective lens 24a, and irradiated as light spot 28a on a recording medium 20 formed in the shape of a disk. The reflected light 29a' of the laser light 29a on the recording medium 20, is transformed into a parallel pencil of rays by the objective lens 24a then reflected at right angles toward a prism 25 by the polarized beam splitter 23a.
Similarly, the laser light 29b emitted from the laser light source 21b in the center-left part of the figure, is irradiated as light spot 28b to the recording medium 20 across the collimating lens 22b, the polarized beam splitter 23b and the objective lens 24b. The reflected light 29b' of the laser light 29b on the recording medium 20, is reflected across the objective lens 24b by the polarized beam splitter 23b toward the prism 25 in the same optical path as the reflected light 29a' corresponding to the laser light source 21a mentioned above.
Meanwhile, the above-mentioned sources of laser light 21a and 21b are arranged such that they generate laser lights of which wavelengths are mutually different, for example a laser light 29a of a long wavelength, and a laser light 29b of a short wavelength. The wavelengths of the reflected lights 29a' and 29b' are mutually different as they correspond respectively to the wavelengths of the laser lights 29a and 29b therefore, the deflection angles on the prism 25 of the reflected lights 29a' and 29b' are different. Accordingly, the deflection angle of the reflected light 29a', corresponding to the laser light 29a of for example a long wavelength (hereinafter referred to as long wavelength reflected light 29a'), is smaller than the deflection angle of the reflected light 29b' corresponding to the laser light 29b of a short wavelength (hereinafter referred to as short wavelength reflected light 29b'). Thereby the long wavelength reflected light 29a' is converged by the focusing lens 26 toward the light receiving element 27a. Meanwhile, as its deflection angle is bigger than the deflection angle of the long wavelength reflected light 29a', the short wavelength reflected light 29b' is converged by the focusing lens 26 toward the light receiving element 27b.
In an arrangement such as mentioned above, as the recording, the reproduction and the like is effectuated simultaneously at different locations on the recording medium 20, the processing time is shorten by half, compared with the optical pickup apparatus with a single laser light source mentioned earlier, and a plurality of informations can be processed at great speed.
However, in the optical pickup apparatus arranged as mentioned above, the members composing the optical system section which processes the reflected lights 29a' and 29b' after they are reflected by the polarized beam splitters 23a and 23b, need to be installed in accordance with the deflection angle at the prism 25. Therefore the miniaturization of the apparatus used to be difficult.
Hence, there is known, as an improvement to compensate for the disadvantage mentioned above, an optical pickup apparatus which, as illustrated in FIG. 5, uses instead of the prism 25 a focusing lens 36 having a property such that the index of refraction varies according to the wavelength of the incident light (wavelength dispersion function). The arrangement leading the reflected lights 39a' and 39b' to the focusing lens 36 is omitted in FIG. 5 as it is the same as in the above-mentioned FIG. 4.
In the optical pickup apparatus provided with an optical system section as shown in FIG. 5, when the reflected lights of different wavelengths proceeding in one and same optical path, for example a long wavelength reflected light 39a' and a short wavelength reflected light 39b', are incident on the focusing lens 36, the focus 40a of the reflected light 39a' and the focus 40b of the reflected light 39b' are focused on the same optical axis at different positions.
For example, the focus 40a of the long wavelength reflected light 39a' is focused farther from the focusing lens 36 on the optical axis than the focus 40b of the short wavelength reflected light 39b'.
The photo-detector 37 is mounted in the neighborhood of, for example like in the present embodiment, the focus 40a of the long wavelength reflected light 39a', which is the farther among the focuses 40a and 40b of the reflected lights 39a' and 39b' from the focusing lens 36.
As a result, the reflected light 39a' is converged in the neighborhood of the photo-detector 37 in a circle of a small diameter and the reflected light 39b', after being converged into focus 40b spreads into a circle of a big diameter in the neighborhood of the photo-detector 37. The above-mentioned photo-detector 37 is composed of a small diameter light receiving element 41 which light receiving face is formed in the shape of a circle of a small diameter, and a big diameter light receiving element 42 which light receiving face is formed in the shape of a ring within which inner diameter is mounted the light receiving face of the small diameter light receiving element 41 in a concentric manner.
Accordingly, the reflected light 39a' converged in a circle of a small diameter is detected by the small diameter light receiving element 41, and the reflected light 39b' spread into a circle of a big diameter is detected by the big diameter light receiving element 42.
In such an arrangement, the prism 25 mentioned earlier becomes unnecessary, and the optical path on and after the polarized beam splitters 23a and 23b can be formed in a straight line. Thereby, the miniaturization of the optical pickup apparatus is facilitated.
However, in the conventional optical pickup apparatus mentioned above, as the small diameter light receiving element 41 mounted in the photo-detector 37 is irradiated simultaneously by the reflected light 39b', spread in a circle of a big diameter, and by the reflected light 39a' converged in a circle of a small diameter, the reflected light 39b' though slightly, contaminates the reflected light 39a'. And as a result the conventional optical pickup apparatus mentioned above used to present the problem that the detection signal detected by the small diameter light receiving element used to contain noise (crosstalk).